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Chapter 11. Winter Operations

Table of Contents
11.1. Aircraft contamination on the ground
11.2. Contaminated runway operations

11.1. Aircraft contamination on the ground

11.1.1. Allowable contamination

The effect of frozen contamination on aircraft take-off performance is unpredictable. Allowable contamination is therefore limited by Airbus to the following cases:

  1. Thin hoar frost on upper fuselage, radome and nacelles

  2. ≤3mm of frost on underside of wing tank area

[FCOM PRO.SUP.91.30]

11.1.2. Removal of contamination

Airframe

Airframe contamination is primarily removed using de-icing fluid.[7] The main exception is the areas forward of the cockpit windows which should be de-iced mechanically.[8] Note also that Airbus recommends removing contamination from the windshield and upper cockpit fuselage before turning on the window heat so as to avoid contamination of critical areas by re-frozen run off.

The airframe de-icing procedure is available in the supplementary information section of the QRH. This procedure prevents the ingress of de-icing fluid through the use of the ditching button. As this closes the outflow valve, all air sources, including ground air sources, must be turned off or disconnected during de-icing. It is permissible to de-ice the aircraft with the engines and/or APU running. With the engines running it is particularly important to maintain communications with the ground personnel so that they can be co-ordinated should an evacuation become necessary. If the APU is running and the fuselage has been sprayed, the APU bleed should remain off for approximately 5 minutes after de-icing to prevent the ingestion of de-icing fluid into the air conditioning system.

Once contamination has been removed, the airframe must remain uncontaminated until airborne. If conditions are such that re-contamination may occur, viscous anti-icing fluids that remain attached to the aircraft are used to provide protection until they are sheared off by high speed airflow during the take-off roll.

Anti-icing fluids have the potential to fail, either by freezing or losing their viscosity and flowing off the aircraft. A fluid's freezing point and viscosity is determined by its chemical makeup and its dilution, dilution being a function of initial dilution[9] and the amount of water absorbed in the process of protecting the aircraft. “Hold over time” tables are provided to allow estimation of the amount of time available before fluid failure occurs for a given combination of fluid type, temperature, intial dilution and precipitation type. The time is given as a range, the shorter time corresponding to “medium“ precipitation and the longer time corresponding to “light” precipitation. It is possible that the wing may be colder than its surroundings due to cold soaked fuel contained within. Therefore if the fuel temperature is below the ambient temperature, the fuel temperature should be used in hold over time calculations.

As anti-ice fluids are designed to shear off the aircraft as airspeed increases they are also susceptible to failure due to high winds and jet blast.

De-icing may be combined with anti-icing or carried out as distinct steps. In a two step process, hold over time begins at the commencement of the anti-icing step.

When taxiing over contaminated areas there is a risk that slush will contaminate the flap mechanisms. For this reason the flaps are kept retracted under these circumstances and will usually also be retracted during de-icing. Extending the flaps will therefore expose unprotected areas to precipitation and should therefore be left until just before takeoff in these conditions.

[FCOM PRO.SUP.91.30, EOMA 8.2.4]

Engines

The engines must not be started until all contamination has been removed. Removal of this contamination is an engineering function, usually involving the use of hot air blowers.

When operating for extended periods in icing conditions it is possible that the fan blades will become contaminated by ice. Airbus provides a fan blade ice shedding procedure which should be applied when ground operations in icing conditions have exceeded 30 minutes. The procedure is to accelerate the engines to 70% N1 for 30 seconds every 30 minutes and additionally just before take-off. If operating in freezing fog, freezing rain or heavy snow an additional run up to 70% with no dwell time should be carried out every 10 minutes.

[EOMB 2.4.91]

11.2. Contaminated runway operations

Runway contamination affects aircraft operations in three ways:

  1. Reduced lateral tyre friction reduces lateral control, thus lighter crosswinds become limiting.

  2. Reduced longitudinal tyre friction reduces the aircraft's ability to stop, affecting both accelerate stop and landing distances.

  3. Contaminant drag caused by displacement and impingement reduces the aircraft's ability to accelerate, affecting all aspects of take-off performance.

Each contaminant type results in a different mix of these factors.

Where less than 25% of the runway is contaminated and/or the contaminant is water, slush or snow with a depth of 3mm or less the effects are negligible enough to simply consider the runway wet. This may, however, not be appropriate if contamination is localised to critical areas of the runway.

Where the contamination exceeds 12.7mm of water or slush, 25.4mm of wet snow or 100mm of dry snow, take-off is prohibited. Both takeoff and landing are prohibited if there is a layer of contaminant on top of a layer of ice or compacted snow since no performance data is available for combinations of contaminants. In addition EOMB 2.1 prohibits takeoff on wet ice.

Between these extremes, take-off and landing are permitted so long as the effects of the contamination are mitigated:

  • Item 1 is dealt with by introducing more restrictive crosswind limitations, these being promulgated in EOMB 2.1 and QRH PER.C

  • Items 2 and 3 are dealt with through the use of more restrictive performance data. This data is available via the EFB's runway condition field. The EFB runway condition field does not have an option for snow, so a table is presented in EOMB 4.6.8 for takeoff and QRH PER.C for landing to transform snow contamination into equivalent slush or water contamination.

In addition, snow clearing operations may have resulted in the build up of snow banks in the proximity of the runway. A diagram in EOMB 4.6.10 defines the maximum snow bank height against distance from the runway that is permitted for take-off.

The captain must be PF for all contaminated runway operations, and all contaminated take-offs should use TOGA thrust.

[EOMB 4.6, EOMB 4.13, EOMB 2.1, QRH PER.C]



[7] “Forced air” de-icing has recently been introduced at certain stations. This may be used in addition to or instead of de-icing fluid under certain circumstances.

[8] De-icing fluid remaining on these areas will run back over the windows and obscure vision during the take-off roll.

[9] Fluids may be applied pre-diluted with water to save expense.